1. Field of the Invention
The present invention concerns a method for generating nanostructures on metal pieces and a device for generating nanostructures.
Nanocrystalline materials are characterized by ultrafine grains, typically with at least one dimension of less than 100 nm. These materials are produced using known methods such as, for example, IGC (inert gas condensation and consolidation), SPD (severe plastic deformation), etc. These methods have the drawback of generating materials that are neither porosity-free nor contamination-free, nor of sufficient size for industrial applications.
The purpose of the method of the invention is to create on the surface of the material a layer of this same material having constituent grains of several tens of nanometers, forming what is commonly called a layer of nanometric microstructures, or nanostructures.
There are conventional peening methods that are known in the prior art. Peening the surface of a material, for example a metal, consists of shooting projectiles of small size, for example balls, at speeds between 5 and 100 mps. In this prior art, the balls are projected by means of a blast of compressed air. In this method of peening, the balls are not immediately reused, and pass through a recycling device before being fed back to the blast nozzle. Moreover, each blast that is incident on the piece is unidirectional, at a given angle for a given surface. In addition, a continuous sweep of the piece is required during the peening in order to obtain a homogeneous surface. Furthermore, the results obtained show that the surface of the treated piece comprises few or no nanostructures. The only advantage of the conventional peening process resides in the fact that it is possible to obtain higher ball speeds than in the generation of nanostructures using ultrasound. In essence, generating nanostructures using ultrasound makes it possible to obtain ball speeds between 5 and 20 mps, whereas pneumatic blast nozzle peening makes it possible to obtain ball speeds between 10 and 100 mps.
2. Description of the Related Art
There is also a method for ultrasonic hardening of metal pieces known from French patent application 2,689,431 or Russian patent application 1,391,135, which consists of setting balls in motion inside an enclosed volume for a given amount of time, using an ultrasound generator. In the method of the French patent application, it is possible to obtain, as a function of the speed, either a given roughness, or a given hardened layer depth. In order to obtain a uniform treatment, the travel speed of the transmitter must conform to a certain value, below which there is strain hardening of the surface and above which the treatment is no longer uniform, meaning that some point on the surface will not have been hit even once. The speeds envisaged in this patent application are no more than several tens of centimeters per second, and the amplitude of the transmitter is 100 μm. Thus, the known operating mode does not make it possible to create a hardened layer without obtaining a nanometric structure to a significant depth.